Wide body commerical aircraft utilize turbofan gas turbine engines for powering the aircraft in flight. The turbofan engines includes a fan powered by a low pressure turbine (LPT) through a low pressure (LP) shaft, and a coaxial compressor powered by a high pressure turbine (HPT) through a high pressure (HP) shaft. The fan and LP shaft have a maximum rotational speed which is substantially less than the maximum rotational speed of the HP shaft for improving propulsion efficiency of the fan in powering the aircraft in flight.
In order to start the engine, a conventional air-driven starter is selectively operatively joined to the HP shaft through a suitable gearbox and is effective for rotating the HP shaft to a suitable speed of about 17% maximum RPM, so that the compressor can provide suitable compressed air to the combustor, which is then mixed with fuel and ignited for generating the combustion gases which power both the HPT and the LPT. The air starter is then suitably disconnected from the HP shaft once the engine is started and both the compressor and fan are being powered by the HPT and the LPT, respectively.
As aircraft turbofan engines become ever larger for producing higher levels of thrust, the compressor and HPT become larger and have larger rotational inertia which must be overcome by the starter for suitably rotating the HP shaft in order to start the engine. Accordingly, a correspondingly larger starter may be used to accelerate the HP shaft during start-up within an acceptable time interval, but, this increases installed weight of the engine in the aircraft and requires a suitably larger auxiliary power unit (APU) to drive the starter.
Furthermore, in the event engine restarting is required during flight of the aircraft, the time interval required to effect engine starting becomes more important and should be as small as possible for obtaining prompt engine start. Prompt air starting is also desirable without the need for increasing the size of the conventional starter or APU.